1. Field of the Invention
The invention generally relates to high-frequency power sources, and more particularly to a multi-subband design technique for a double-barrier quantum-well intrinsic oscillator defined by the double-barrier heterostructures of a resonant tunneling diode.
2. Description of the Related Art
Within this application several publications are referenced by Arabic numerals within brackets. Full citations for these and other publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications as well as the heretofore referenced patents in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the invention and illustrating the general state of the art.
Double-barrier quantum-well structures, such as those described in U.S. Pat. No. 5,844,253 issued to Kim et al. on Dec. 1, 1998, are well known in the semiconductor industry. The search for compact solid-state based, high-frequency power sources has been an important research subject for many years[1]. For many years, resonant tunneling diodes (RTD) have been treated as possible high frequency power sources[2]. However, as it is well known, the traditional implementation of a RTD has not been successful as a power source at terahertz (THz) frequencies[3, 5, 6]. Indeed, the output power of a RTD is on the order of μ watts at operation frequencies near 1 THz[5]. This failing is contributed by the extrinsic design manner of the oscillator that utilizes external circuit elements to induce the oscillation. This failing of the “traditional” RTD-based oscillator is tied directly to the physical principles associated with its implementations. In fact, the f−2 law indicates that it is impossible to get higher output power at terahertz frequencies for a single device utilized in an extrinsic design manner[2].
In contrast to the extrinsic design of RTD oscillators, the intrinsic design of RTD oscillators makes use of the microscopic instability of RTDs directly[3, 7]. This type of an approach avoids the drawbacks associated with the extrinsic implementation of RTDs. It is believed that if the dynamics surrounding the intrinsic oscillation can be understood and controlled, RTD sources based on the self-oscillation process should yield milliwatt levels of power in the THz regime[3]. However, the exact origin of the intrinsic high-frequency current oscillation has not yet been fully established. The transport dynamics in RTDs is governed by the quantum mechanical tunneling process that occurs through a quantum-well that is formed by a double-barrier heterostructure. However, the lack of knowledge related to the origin of the intrinsic instabilities in double-barrier quantum-well structures (DBQWSs) directly hampers realizing an optimal design (device and circuit) of a RTD-based oscillator[4]. Thus, it is extremely important to understand the creation mechanism of the intrinsic instability in DBQWSs.
Historically, Ricco and Azbel suggested in their qualitative arguments, that intrinsic oscillation exists in a double barrier structure fur the case of one-dimensional transport[8]. Their theory attributed the instability to a process that cycled in and out of resonance. Specifically, when the energy of the incoming electrons matched the resonance energy, the tunneling current then charged the potential well and lifted its bottom, thus driving the system away from resonance. The ensuing current decrease (i.e., associated with the off-resonance) then reduced the charge in the well, bringing the system back to resonance, and a new cycle of oscillation commences. According to such a theory, there should be current oscillation at the resonance bias. However, numerical simulation results contradict this simple theory[9, 12, 15].
In another theory, it was suggested that the nonlinear feedback caused by stored charges in the quantum well was responsible for the creation of the current oscillation[16]. However, this phenomenological theory does not explain why the nonlinear feedback caused by stored charge in the quantum-well at bias voltages lower than those associated with resonance does not lead to current oscillation[23]. In subsequent studies of RTDs, Jensen and Buot observed intrinsic oscillations in their numerical simulations of DBQWSs[12]. However, this initial work fails to provide underlying explanations of the oscillation mechanism. Recently, Woolard et al. suggested that the current oscillation might be caused by the charge fluctuation near the emitter barrier of the RTD[17]. However, the cause of the charge oscillation and how the charge oscillation affects the electronic resonant tunneling were not clearly indicated. Hence, the origin of intrinsic oscillation has eluded revelation for many years.
Furthermore, very high frequency electron dynamics in tunneling structures is of fundamental importance to nanoelectronics. Experimental investigations of similar time-dependent processes are also receiving more attention[13]. However, to date, there has not been a completely conclusive demonstration of intrinsic oscillations in RTDs. Hence, the development of an accurate fundamental technique that provides insight into the catalyst of the intrinsic oscillation is a key first step for the successful design of an RTD-based oscillator.
In earlier works, a new theory was presented that provided a basic idea for the origin of the intrinsic oscillation in DBQWSs[23] (hereinafter referred to as Paper I). This theory reveals that the current oscillation, hysteresis, and plateau-like structure in a I-V curve are closely related to the quantum mechanical wave/particle duality nature of the electrons. In addition, these effects were shown to be a direct consequence of the development and evolution of a dynamic emitter quantum well (EQW), and the ensuing coupling of the quasi-discrete energy levels that are shared between the EQW and the main quantum-well (MQW) formed by the DBQWS. Through this new understanding of the dynamic behavior of the RTD, it was possible to qualitatively predict the existence of an oscillation. However, while this initial description was able to self-consistently explain all the physical phenomena related to the intrinsic oscillation, it could not provide quantitative design rules.
Therefore, due to the limitations of the conventional techniques, processes, and theories, there remains a need for a multi-subband model for describing the electron dynamics in DBQWSs and an approach for the design and development of a semiconductor-based signal source at very high frequencies.